Negative electrode active material for secondary battery, conductive composition for secondary battery, negative electrode material comprising same, negative electrode structure and secondary battery comprising same, and method for manufacturing same

ABSTRACT

The present invention relates to a negative electrode active material for a secondary battery, a conductive composition for a secondary battery, a negative electrode material including the same, a negative electrode structure and secondary battery including the same, and a method for manufacturing the same. The present invention includes: a silicon particle; and an amorphous surface layer formed on the surface of the silicon particle. According to the present invention, the negative electrode structure is formed of a composite of a silicon particle and carbon or lithium ion, the oxygen contents of the solid electrolyte and silicon particles are low, and thus aggregation of silicon particles is inhibited. Therefore, in the event of using the negative electrode structure in a negative electrode, a power storage device such as a lithium secondary battery may have high energy density, high output density, and a longer charging/discharging life cycle.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 14/441,107, filed May 6, 2015, which claims priority toPCT/KR2013/012349, filed on Dec. 27, 2013, which claims priority toKorean Patent Application No. 10-2012-0155529, filed on Dec. 27, 2012,and all the benefits accruing therefrom under 35 U.S.C. § 119, thedisclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a negative electrode active materialfor a secondary battery, a conductive composition for a secondarybattery, a negative electrode material including the same, a negativeelectrode structure and secondary battery including the same, and amethod for manufacturing same, and more specifically, to a negativeelectrode active material including a silicon particle having anamorphous surface layer, or a silicon-carbon composite, a conductivecomposition for a secondary battery, which includes carbon, a negativeelectrode material including the same, a negative electrode structureand secondary battery including the same, and a method for manufacturingthe same.

BACKGROUND ART

Recently, the amount of CO₂ gas contained in the atmosphere hasincreased, which increases the occurrence of the greenhouse effectcausing global warming. Effects of the air pollution due to substancesincluding CO₂, NO_(x), hydrocarbon and the like emitted from automobilesused as transportation means negatively affects the health of people.From the viewpoint of the increase in prices for energy such as crudeoil and environmental protection, much expectation has been placed on asmart grid which is a system that optimizes the balance of need forelectric power by network management of electric power in a hybridvehicle, which combines an electric motor and an engine operated byelectricity stored in a power storage device, an electric vehicle, andan electric power generation facility, which have high energyefficiency.

Further, even in the information communication field, an informationterminal such as a smart phone has rapidly infiltrated into the societydue to the ease of exchanging information and sending messages. Underthese circumstances, in order to enhance the performance of a smartphone, a hybrid vehicle, an electric vehicle, a smart grid and the likeand reduce production costs, it has been expected to develop a powerstorage device such as a capacitor or a secondary battery, whichcombines a high electric power density and a high energy density, and along service life.

Among currently commercially available devices as the power storagedevice, a device having the highest energy density is a lithium ionsecondary battery using carbon such as graphite in the negativeelectrode and compounds of lithium and a transition metal in thepositive electrode. However, since the negative electrode is composed ofa carbon material in the “lithium ion battery”, only up to ⅙ of lithiumatoms per carbon atom may be theoretically intercalated. For thisreason, it is difficult to achieve a new high capacitance battery, andthere is a need for a new material for the negative electrode forachieving high capacity. In addition, even though the “lithium ionbattery” has high energy density, and thus is expected to be a powersource for a hybrid vehicle or an electric vehicle, there is a problemin that the “lithium ion battery” cannot release a sufficient amount ofelectricity due to high internal resistance of the battery during arapid discharge, that is, has small output density. For that reason,there is need for the development of a power storage device having highoutput density and high energy density.

In order to satisfy these demands, studies have been conducted on tin orsilicon and an alloy thereof, which may store and release a largeramount of lithium ions than graphite. Tin or silicon may storeelectrochemically a larger amount of lithium ions, but expands in volumeby about 4 times and causes pulverization when expansion and contractionoccur due to repeating charge and discharge, thereby causingdeterioration in performance of the battery. In order to prevent theaforementioned pulverization, attempts have been conducted for extendingthe service life of the negative electrode of the battery by grindingsilicon or silicon alloys into particulates.

As a method of making the silicon material a particulate, there is amechanical grinding method, and as a device which may grind the siliconmaterial into a particle size of sub-micron or less, there is a wetbeads mill which is a kind of media mill. Grinding by the wet beads millhas problems of (1) reducing the content of oxygen in silicon powderwhich is a raw material, (2) suppressing oxidation during the grinding,(3) suppressing ground particles from re-aggregating, and (4)suppressing particles from aggregating when the silicon material isground and then dried.

Patent Documents 1, 2, 3, 4 and 5 disclose methods of using beads millin grinding silicon or silicon alloys.

Patent Document 1 discloses that (i) Si particles having an averageparticle diameter (D₅₀) of 0.05 to 5 μm are prepared by a wet grindingusing a beads mill, (ii) as a solvent to be used, toluene, xylene,mesitylene, methyl naphthalene, creosote oil and the like which areinert to Si are used, and (iii) a wet-mixing heat treatment is performedby adding ground Si particles and a carbon material or a precursorthereof. However, there are problems in that when Si particles are mixedwith a carbon material or a precursor thereof before the heat treatment,an oxygen source has not completely been removed, and accordingly, Siparticles are oxidized, Si particles having a particle diameter of 0.05to 5 μm aggregate in a heat treatment, and accordingly, thecharging/discharging life cycle of the battery is not long, and thelike.

Patent Document 2 discloses a beads mill as a wet grinding device whichforms an alloy from a mixture of Si powder and transition metal by amechanical alloying method, and grinds the alloy into particles havingan average particle diameter (D₅₀) of 0.50 to 20 μm. Patent Document 2also discloses that it is possible to use a non-protic solvent such ashexane, acetone, and n-butyl acetate, and a protic solvent such aswater, methyl alcohol, ethyl alcohol, 1-propyl alcohol, 2-propylalcohol, 1-butyl alcohol, and 2-butyl alcohol as a dispersion mediumused in the wet grinding. However, Patent Document 2 does not disclose amethod of grinding the silicon powder into particles having an averageparticle diameter (D₅₀) of less than 0.5 μm.

Patent Documents 3, 4, and 5 disclose that a Si—Sn—Cu alloy powder isground into particles having an average particle diameter of up to 0.28μm by using a beads mill grinding using zirconia beads as beads andisopropyl alcohol as a medium. However, there are problems in that dueto pulverization, contact resistance between particles is increased, andcharging/discharging efficiency deteriorates, and characteristicsexpected by using particulates have not been exhibited. Furthermore, asan attempt to increase the electrochemical reaction efficiency byincreasing conductivity even in the negative electrode, an attempt toincrease the conductivity by adding carbon nanotube and carbon nanofiberin small amounts has also been conducted, but carbon nanotube and carbonnanofiber aggregate with each other, so that it is difficult toefficiently disperse carbon nanotube and carbon nanofiber, the coastsare high, and it is difficult to increase the content of carbon nanotubeand carbon nanofiber in the negative electrode.

CITATION LIST Patent Document

(Patent Document 1) 1. Official gazette of Japanese Patent ApplicationLaid-Open No. 2008-112710

(Patent Document 2) 2. Official gazette of International Publication No.WO2006/129415

(Patent Document 3) 3. U.S. Pat. No. 7,141,187

(Patent Document 4) 4. U.S. Pat. No. 7,316,792

(Patent Document 5) 5. U.S. Pat. No. 7,803,290

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide a method formanufacturing a silicon material which may electrochemically store andrelease a large amount of lithium as a negative electrode activematerial for a power storage device, which has high energy density.

In particular, the present invention has been made in an effort toprovide a method for manufacturing a slurry in which silicon particlesfor manufacturing a slurry for forming an electrode layer, whichsuppresses a surface oxidation film of silicon particles from beingproduced and silicon particles from aggregating, are dispersed in amethod for manufacturing an electrode structure for a negative electrodeof a power storage device, such as a lithium secondary battery having anactive material layer composed of a composite of silicon particles andcarbon (a secondary battery using oxidation and reduction reactions oflithium ions). The power storage device herein includes a capacitor, asecondary battery, a device which combines a capacitor with a secondarybattery, and a device to which a power generation function is alsoimparted.

Technical Solution

A first exemplary embodiment of the present invention may be a negativeelectrode active material for a secondary battery, including a siliconparticle and an amorphous surface layer formed on the surface of thesilicon particle, in which the material is manufactured by manufacturinga slurry including the silicon particle through a dry grinding and a wetgrinding, and then mixing and dispersing a material which forms theamorphous surface layer in the slurry.

A secondary exemplary embodiment of the present invention may be anegative electrode active material for a secondary battery, including asilicon particle, a carbon particle with a graphene structure, and asilicon-carbon composite including fibrous carbon and carbon black, inwhich the silicon particle is in contact with one or more selected fromthe group consisting of a carbon particle with a graphene structure,fibrous carbon, and carbon black.

A third exemplary embodiment of the present invention may be a methodfor manufacturing a negative electrode active material, the methodincluding: a dry grinding process of performing a dry grinding on astarting material under an inert atmosphere to prepare a dry groundproduct, and a wet grinding and dispersing process of performing a wetgrinding on the dry ground product and dispersing the dry ground productto manufacture a slurry.

A fourth exemplary embodiment of the present invention may be aconductive composition for a secondary battery, in which carbon isdispersed in a non-protic solvent having no proton donor ability.

A fifth exemplary embodiment of the present invention may be a methodfor manufacturing a conductive composition for a secondary battery, themethod including: a process of mixing an additive with a non-proticsolvent to prepare a solution; and a dispersing process of adding acarbon material to the solution to disperse the material. The dispersingprocess may be performed by a beads mill.

A sixth exemplary embodiment of the present invention may be a negativeelectrode material for a secondary battery, including a solvent, anegative electrode active material, the conductive composition of thethird exemplary embodiment, and a binder, in which the negativeelectrode active material includes one or more selected from the groupconsisting of the negative electrode active material of the firstexemplary embodiment and the negative electrode active material of thesecond exemplary embodiment.

A seventh exemplary embodiment of the present invention may be a methodfor manufacturing a negative electrode material for a secondary battery,the method including: preparing a negative electrode active materialaccording to the third exemplary embodiment; preparing a conductivecomposition according to the fifth exemplary embodiment; andmanufacturing a slurry by mixing the negative electrode active material,the conductive composition and a binder with a solvent, in which thenegative electrode active material includes one or more selected fromthe group consisting of the negative electrode active material of claim1 and the negative electrode active material of claim 7.

An eighth exemplary embodiment of the present invention may be anegative electrode structure including a conductive metal and a negativeelectrode material layer formed on the conductive metal, in which thenegative electrode material layer includes the negative electrodematerial of the sixth exemplary embodiment, and the conductive metal mayinclude one or more selected from the group consisting of copper andaluminum.

A ninth exemplary embodiment of the present invention may be a methodfor manufacturing a negative electrode structure, the method including:preparing a negative electrode material according to the seventhexemplary embodiment; applying the prepared negative electrode materialon a conductive metal; and subjecting the applied negative electrodematerial to heat treatment.

A tenth exemplary embodiment of the present invention may be a secondarybattery including: the negative electrode structure of the eighthexemplary embodiment; a separator; a positive electrode structure; and acurrent collector.

An eleventh exemplary embodiment of the present invention may be amethod for manufacturing a secondary battery, the method including:preparing a negative electrode structure according to the ninthexemplary embodiment; and stacking the prepared negative electrodestructure, a separator, and a positive electrode structure.

Advantageous Effects

According to the present invention, since the negative electrode activematerial manufactured using a slurry in which a silicon particle isdispersed is formed of a composite of a silicon particle and carbon or alithium ion solid electrolyte, and silicon particles are suppressed fromaggregating due to a low content of oxygen in silicon particles, a powerstorage device such as a lithium secondary battery may have performancesof high energy density, high output density and a longercharging/discharging life cycle by using the negative electrode activematerial in a negative electrode.

Further, the method for manufacturing a negative electrode activematerial according to the present invention is excellent in massproduction that the negative electrode active material may beinexpensively manufactured. In addition, the electronic conductivity ofa secondary battery negative electrode may be enhanced by adding theconductive composition according to the present invention during themanufacture of the negative electrode, and accordingly, performance ofthe battery may be enhanced.

DESCRIPTION OF DRAWINGS

FIG. 1 is schematic views (a) and (b) of a silicon particle included ina negative electrode active material according to an exemplaryembodiment of the present invention, a SEM photograph (c) of the siliconparticles, and TEM photographs (d) of the silicon particles.

FIG. 2 is schematic views of a silicon-carbon composite included in thenegative electrode active material according to an exemplary embodimentof the present invention.

FIG. 3 is a flowchart illustrating a process of manufacturing a negativeelectrode active material according to an exemplary embodiment of thepresent invention.

FIG. 4 is a schematic view illustrating a device system used in themanufacture of a negative electrode active material according to anexemplary embodiment of the present invention.

FIG. 5 is a schematic view of a conductive composition for a secondarybattery according to an exemplary embodiment of the present invention.

FIG. 6 is a cross-sectional view of a negative electrode structuremanufactured according to an exemplary embodiment of the presentinvention.

FIG. 7 is a flowchart illustrating a process of manufacturing a negativeelectrode structure manufactured according to an exemplary embodiment ofthe present invention.

FIG. 8 is a schematic cross-sectional view of a power storage devicemanufactured according to an exemplary embodiment of the presentinvention.

FIGS. 9 to 11 are schematic views of a coin-type cell, a laminate cell,and a cylinder-type cell, which are examples of a power storage devicemanufactured according to an exemplary embodiment of the presentinvention.

BEST MODE

Hereinafter, preferred exemplary embodiments of the present inventionwill be described with reference to the accompanying drawings. Exemplaryembodiments of the present invention may be modified in various forms,and the scope of the present invention is not limited to the exemplaryembodiments to be described below. Further, exemplary embodiments of thepresent invention are provided so as to more completely describe thepresent invention to the person skilled in the art. Accordingly, theshape, the size and the like of elements illustrated in the drawings maybe exaggerated for a more clear description, and elements represented bythe same reference numerals in the drawings are the same elements.

FIG. 1 is schematic views (a) and (b) of a silicon particle included ina negative electrode active material according to an exemplaryembodiment of the present invention, a SEM photograph (c) of the siliconparticles, and TEM photographs (d) of the silicon particles. FIG. 2 isschematic views of a silicon-carbon composite included in the negativeelectrode active material according to an exemplary embodiment of thepresent invention. FIG. 3 is a flowchart illustrating a process ofmanufacturing a negative electrode active material according to anexemplary embodiment of the present invention. FIG. 4 is a schematicview illustrating a device system used in a process of manufacturing anegative electrode active material according to an exemplary embodimentof the present invention. FIG. 5 is a schematic view of a conductivecomposition for a secondary battery according to an exemplary embodimentof the present invention. FIG. 6 is a cross-sectional view of a negativeelectrode structure manufactured according to an exemplary embodiment ofthe present invention. FIG. 7 is a flowchart illustrating a process ofmanufacturing a negative electrode structure manufactured according toan exemplary embodiment of the present invention. FIG. 8 is a schematiccross-sectional view of a power storage device manufactured according toan exemplary embodiment of the present invention. FIGS. 9 to 11 areschematic views of a coin-type cell, a laminate cell, and acylinder-type cell, which are examples of a power storage devicemanufactured according to an exemplary embodiment of the presentinvention.

Referring to FIG. 1, a first exemplary embodiment of the presentinvention may be a negative electrode active material including asilicon particle 100 and an amorphous surface layer 101 formed on thesurface of the silicon particle 100.

The silicon particle 100 may be a powder obtained by grinding a siliconelement bulk material, and the average particle diameter of the siliconparticle 100 may be 5 to 200 nm. When the average particle diameter ofthe silicon particle 100 is less than 5 nm, the size of the oxidationsurface area is increased, so that the performance as a negativeelectrode active material may be lower than a designed value, and whenthe average particle diameter thereof is larger than 200 nm, anpulverized surface with respect to a change in volume accompanied bycharging/discharging may steadily occur, thereby leading todeterioration in service life characteristics.

The amorphous surface layer 101 may be formed on the surface of thesilicon particle 100, and the amorphous surface layer 101 may includeamorphous carbon.

Since the silicon particle 100 is blocked from being in direct contactwith the outside by the amorphous surface layer 101 formed on thesurface of the silicon particle 100, the surface of the silicon particle100 may be suppressed from being oxidized. In contrast, the surfacelayer 101 includes carbon, and thus may be used as a carbon precursorfor carbonization or graphitization, which may contribute to electronicconductivity of the negative electrode. In conclusion, by preventing thesurface of the silicon particle 100 from being oxidized, and forming thesurface layer 101 including carbon thereon, the electronic conductivityof the negative electrode may be enhanced through carbonization orgraphitization of the carbon precursor through firing.

The thickness of the amorphous surface layer 101 may be 1 to 10 nm. Whenthe thickness of the amorphous surface layer 101 is less than 1 nm, thesurface layer 101 is so thin that it is too thin to prevent the siliconparticle 100 from being oxidized, and when the thickness is more than 10nm, lithium ions may be inhibited from being inserted into the activematerial.

The primary particle has a long diameter (a) >a small diameter (b) >athickness (d), the long diameter may be 50 to 300 nm, the small diameter30 to 200 nm, and the thickness may be 10 to 50 nm.

Further, FIG. 1 illustrates a scanning electron microscope photograph(c) of silicon particles coated with amorphous carbon and transmissionelectron microscope photographs (d) thereof. FIG. 1(c) is a SEMphotograph of silicon particles milled by zirconia beads having adiameter of 0.1 mm. The two upper photographs of FIG. 1(d) are TEMphotographs of silicon particles milled by zirconia beads having adiameter of 0.2 mm, and the two lower photographs are TEM photographs ofsilicon particles milled by zirconia beads having a diameter of 0.03 mm.According to the photographs, it can be confirmed that the amorphouscarbon layer 101 is formed on the surface of the silicon particle 100.

Referring to FIGS. 2(a) and 2(b), the second exemplary embodiment of thepresent invention is a negative electrode active material including asilicon-carbon composite 110 including one or more selected from thegroup consisting of a silicon particle 111, a carbon particle 112 with agraphene structure, fibrous carbon 113, and carbon black 114, and thesilicon particle 111 may be a negative electrode active material whichis in contact with one or more selected from the group consisting of thecarbon particle 112 with a graphene structure, the fibrous carbon 113,and the carbon black 114.

In the present exemplary embodiment, it may be understood that carbon iscomplexed with the silicon particle 111 which is a main material of thenegative electrode active material in various forms. That is, variousforms of carbon, that is, the carbon particle 112 with a graphenestructure, the fibrous carbon 113, and the bead-like carbon black 114are complexed with the silicon particle 111. A complex effect may bemaximized by complexing various forms of carbon with the siliconparticle.

Referring to FIG. 2(c), the silicon particle 100 may be a powderobtained by grinding silicon, and the average particle diameter of thesilicon particle 100 may be 5 to 200 nm. The amorphous surface layer 101may be formed on the surface of the silicon particle 100, the amorphoussurface layer 101 may include an amorphous carbon layer, and thethickness of the amorphous surface layer 101 may be 1 to 10 nm.

The description of the silicon particle 100 and the surface layer 101thereof is the same as that of the first exemplary embodiment.

The carbon particle 112 with a graphene structure may mean a particle inwhich a carbon particle has a graphene structure. The carbon particle112 with a graphene structure may include one or more selected from thegroup consisting of graphene and graphite. The average particle diameterof the carbon particle 112 with a graphene structure may be 300 nm to 10um.

The average diameter of the fibrous carbon 113 may be 10 to 200 nm. Thefibrous carbon particle 113 may include one or more selected from thegroup consisting of carbon nanofiber and carbon nanotube.

The carbon black 114 has a primary particle having a bead-like connectedstructure, the average particle diameter of the primary particle may be10 to 80 nm, and the crystal size of the primary particle may be 2 to 5nm. The average particle diameter of the silicon-carbon composite 110may be 5 to 20 um.

The silicon particle 111 may be in contact with one or more selectedfrom the group consisting of the carbon particle 112 with a graphenestructure, the fibrous carbon 113 and the carbon black 114. Since carbonsuch as the carbon particle 112 with a graphene structure, the fibrouscarbon 113, and the carbon black 114 are electronically conductive,electronic conductivity between silicon particle and silicon particlemay be enhanced due to the presence of one or more of those carbons,which are in contact with the silicon particle 111, thereby enhancingthe performance of the battery.

The silicon-carbon composite 110 may further include a lithium ion solidelectrolyte particle in addition to the various carbons, and the siliconparticle may be in contact with the lithium ion solid electrolyteparticle. In this case, transition of lithium ions may easily occur onthe surface of the silicon particle when a power storage device isoperated.

A lithium ion inorganic solid electrolyte may include a sulfur-basedamorphous electrolyte represented by Li₂S—P₂O₅, a sulfur-containingglass, lithium nitride (Li₃N), a material with a NASICON crystalstructure represented by Li_(1+x+y)Al_(x)Ti_(2−x)Si_(y)P_(3−y)O₁₂(x=0.3, y=0.2), a material with a garnet structure represented byLi₇La₃Zr₂O₁₂, and a germanium-phosphorus-sulfur compound represented byLi₁₀GeP₂S₁₂.

Referring to FIGS. 3 and 4, the third exemplary embodiment of thepresent invention may be a method for manufacturing a negative electrodeactive material, the method including: a dry grinding process ofperforming a dry grinding on a starting material under an inertatmosphere to prepare a dry ground product, and a wet grinding anddispersing process of performing a wet grinding on the dry groundproduct and dispersing the dry ground product to manufacture a slurry.At least one of the dry grinding process and the wet grinding anddispersing process may be performed by a beads mill process. For thecoherence of the process, it is preferred that all the processes areperformed by the beads mill process.

First, a dry grinding may be performed on a starting material under aninert atmosphere to prepare a ground product (a dry grinding process, afirst process). The starting material is not particularly limited aslong as the material may be used as a negative electrode material. Thestarting material may include a silicon powder, and may also includeanother material in addition to silicon. Coal-tar pitch is adopted as anexample of the additive, and graphite is adopted as an example of theadded material, but the present invention is no limited thereto. FIG.3(a) illustrates the case where the added material graphite is mixedwith the starting material silicon. FIG. 3(b) illustrates the case whereonly the silicon powder is used as the starting material, and the addedmaterial graphite is added after the dry grinding and before the wetgrinding.

The average particle diameter of the starting material silicon powdermay be several mm or less. When the average particle diameter thereof islarger than this value, time needed for the grinding may be prolonged,thereby leading to a drop in productivity. The average particle diameterof the silicon particle obtained in the first process is preferablyseveral μm or less, more preferably in a range of 1 to 10 μm. In thisprocess, a beads mill is used as the dry grinding device, but othermedia mills such as a jet mill or a vibrating mill may be used.

The starting material may be prevented from being oxidized by performinga dry grinding on the starting material under an inert atmosphere. Sinceheat at high temperature may be generated in the dry grinding process,the particle to be ground may be easily oxidized when the particle is incontact with oxygen in the air, and the dry grinding may be performedunder an inert atmosphere to block the starting material to be groundfrom being in contact with oxygen, thereby preventing the startingmaterial from being oxidized. The inert atmosphere may be, but is notlimited to, a nitrogen gas atmosphere, or an argon gas atmosphere.

When silicon is used as the starting material, the surface of thesilicon particle may be prevented from being oxidized. This is becausewhen the surface of the silicon particle is oxidized, the oxidationlayer is electrically insulated, and accordingly, electronicconductivity of the negative electrode manufactured by using theoxidation layer deteriorates, and as a result, the performances of thenegative electrode and the power storage device may deteriorate.

Next, the dry ground product may be mixed with a solvent in which theadditive is dissolved, and may be ground and dispersed with a wet beadsmill to prepare a slurry (a wet grinding and dispersing process, asecond process, and a third process). When the primary particle of thestarting material is less than 1 μm, the dry grinding process (the firstprocess) may be omitted, and the wet grinding process (the secondprocess) may be initially performed. In FIG. 3, the wet grinding processis performed in two steps of the second process and the third process,but may also be performed in a multi-step process composed of three ormore steps. This is because it is preferred to grind silicon particlesinto a desired size by gradually decreasing the size of the beads andgrinding the silicon particles in a multi-step process composed of twoor more steps. Specifically, the silicon ground in the dry grindingprocess (the first process) may be mixed with a solution in which theadditive coal-tar pitch is dissolved in a non-protic solvent and beground for a predetermined time by using a first wet beads mill in thesecond process, and subsequently, may be further ground by a wet beadsmill using beads with a smaller size in the third process, therebyobtaining a slurry in which the silicon particles are dispersed.

The dry ground starting material needs to be transported to a wetgrinding process while not being in contact with oxygen. This is forpreventing the surface of the dry ground starting material under aninert atmosphere from being oxidized.

By appropriately selecting a solvent which is a medium used in the wetgrinding, the surface of silicon may be suppressed from being oxidizedduring the grinding, and silicon particles may also be suppressed fromaggregating. Furthermore, by selecting an additive and an added materialwhich are easily dissolved in the solvent and do not adversely affectthe performance of a power storage device even though the additive andthe added material remain during the firing, the ground siliconparticles may be suppressed from re-aggregating. It is possible to use asolvent having no proton donor ability (a non-protic solvent) and asolvent having proton donor ability (a protic solvent) as the solventused in the second and third processes, but it is more preferred to usethe non-protic solvent because in the case of using a non-proticsolvent, it is more difficult for the surface of silicon to be oxidizedduring the grinding.

Examples of the non-protic solvent include one or more selected from thegroup consisting of a cyclic hydrocarbon, an aromatic compound, aketone-based solvent, an ether-based solvent, an amide-based solvent,and a nitrile-based solvent, and among them, a cyclic hydrocarbon, anaromatic compound, and a ketone-based solvent are most preferred. Themost preferred specific examples of the non-protic solvent includeN-methyl-2-pyrrolidone, γ-butyrolactone, N,N-dimethylacetamide,1,3-dimethyl-2-imidazolidine, and cyclohexane. The additive is notparticularly limited as long as the additive is a material which issoluble or easily dispersed in a medium used in the wet grinding andcracking, and is easily carbonized during the firing process under aninert atmosphere. In FIG. 3, the additive coal-tar pitch is added beforethe second process, but may be added before the first process, beforethe third process, or before the fourth process. Instead of coal-tarpitch, petroleum pitch or a polycyclic aromatic hydrocarbon may also beused. Coal-tar pitch and petroleum pitch are inexpensive, and thus aremost preferred. An example of the polycyclic aromatic hydrocarboncorresponds to naphthalene, anthracene, phenanthrene, naphthacene,pyrene, triphenylene, chrysene, pentacene, benzopyrene, corannulene,coronene, ovalene, and the like. The amount of additive added ispreferably in a range of 0.05 to 5 parts by weight, more preferably in arange of 0.2 to 3 parts by weight, and most preferably in a range of 0.5to 2 parts by weight, based on 100 parts by weight of silicon. Theadditive has an effect that the additive is dissolved in a non-proticsolvent to generate a newly ground surface of silicon, andsimultaneously the surface is covered with the solution to suppress theaggregation through electrostatic repulsion or hydrophobic repulsion.

When the protic solvent is used as a solvent of the wet beads mill,there is an advantage in that problems such as environmentalcontamination are minimal. As the protic solvent, water and alcohol arepreferred, and as the additive used in this case, it is preferred to usea polyvinyl pyrrolidone having a ketone or ether structure, or one ormore polymers of a carboxymethylcellulose sodium salt and a group ofpolyvinyl alcohols. The amount of additive added is preferably in arange of 0.05 to 5 parts by weight, more preferably in a range of 0.1 to3 parts by weight, and most preferably in a range of 0.5 to 2 parts byweight, based on 100 parts by weight of the solvent. When silicon isground in the solvent in which the additive is dissolved, silicon may bemore easily bonded to adjacent atoms due to the unsaturated bond(dangling bond) of the silicon atom, which is formed on the surface ofthe ground silicon particle, and accordingly, (1) it is possible toprevent ground silicon particles from re-aggregating and uniformlydisperse ground silicon particles, (2) it is possible to suppress anoxidation film from being formed on the surface of silicon particle, and(3) it is possible to enhance the dispersion of silicon particles in thesilicon particle-carbon composite, and form the interface of siliconwith a carbon material well. Further, the simultaneously ground addedmaterial may also maintain a more stable dispersion state than thesolvent in which the additive is dissolved. Accordingly, in the thirdprocess, it is possible to obtain silicon and a slurry which is moreuniformly dispersed than the added material.

The particle diameter of the silicon particle introduced into the wetbeads mill in the second process is preferably several μm or less, morepreferably in a range of 1 to 10 μm. The wet grinding process may beperformed until the average particle diameter of the starting materialbecomes 3 nm to 200 nm. The average particle diameter of the siliconparticle obtained in the third process is preferably in a range of 3 to200 nm, more preferably in a range of 10 to 200 nm, and most preferablyin a range of 20 to 100 nm. When the average particle diameter of thestarting material is less than 5 nm, aggregation among the molecules ofthe starting material may occur, and accordingly, dispersion may not beachieved well in the subsequent dispersion process, and when thediameter thereof is more than 200 nm, the particles of the startingmaterial are too large to achieve dispersity sufficient to be used as anegative electrode active material.

The wet beads mill used in the present invention is preferably a beadsmill which enables continuous grinding provided with a circulation tank.A material for the beads used in the beads mill is preferably zirconia,alumina, silicon nitride, and titania. In particular, zirconia oralumina has high grinding capacity due to high hardness, and thus ispreferred because zirconia or alumina remains only in a small amounteven while being used in the manufacture of an electrode or a battery inthe subsequent process, and thus has almost no adverse effect, and thediameter thereof is preferably 2 mm or less, and more preferably in arange of 0.03 to 0.8 mm in the wet beads mill.

The added material may be a material which is not dissolved in a mediumused in the wet grinding. When subjected to the dispersing process, theadded material is branched, and thus may be present while beinguniformly dispersed in the slurry. The added material may include one ormore selected from the group consisting of graphite, graphene, carbonnanotube, carbon fiber, amorphous carbon, and a lithium ion inorganicsolid electrolyte. Preferably, the added material may be graphite,graphene, carbon nanofiber, carbon nanotube, and an inorganic solidelectrolyte of lithium ions. The inorganic solid electrolyte of lithiumions is more preferably a so-called “ceramic electrolyte” such as metal,oxide, carbide, a boron compound, a sulfide, and a phosphoric acidcompound, and specific examples of the inorganic solid electrolyte oflithium ions include a sulfur-based amorphous electrolyte represented byLi₂S—P₂O₅, a sulfur-containing glass, lithium nitride (Li₃N), a materialwith a NASICON crystal structure represented byLi_(1+x+y)Al_(x)Ti_(2−x)Si_(y)P_(3−y)O₁₂ (x=0.3, y=0.2), a material witha garnet structure represented by Li₇La₃Zr₂O₁₂, and agermanium-phosphorus-sulfur compound represented by Li₁₀GeP₂S₁₂.

In FIG. 3, an added material (for example, graphite) is added before orafter the first process, but a slurry of an added material, which isfinely ground and dispersed in advance in a separate process, may alsobe added to the second process to the fourth process. The added materialhas a surface potential different from that of silicon, and thus maysuppress silicon particles from aggregating. Further, particles of theadded material may be present while being more uniformly dispersed amongsilicon particles by adding the added material (graphite) during thegrinding of the silicon particles, and when an electrode structure isformed using the same, it is possible to complement electronicconduction among silicon particles, or conduction of lithium ions intosilicon particles.

By grinding silicon in a solvent (medium) containing a carbon source (anadditive or an added material), ground silicon particles may besuppressed from re-aggregating and an insulating oxidation film may besuppressed from being formed on the surface of silicon particle, andwhen a negative electrode is manufactured, conductivity may bemaintained due to reduction in the oxidation film present among siliconparticles, and electronic conductivity among silicon particles, that is,electronic conductivity of the negative electrode may be enhancedthrough carbonization or graphitization of a carbon source through anadditional firing process, and as a result, performance of the batterymay be enhanced. Further, when silicon is ground in a solvent includinga solid electrolyte, lithium ions may be easily inserted into thesurface of silicon particle when a power storage device is operated.

A process of irradiating ultrasonic waves to perform the treatment mayalso be included in the first to third processes. The ultrasonictreatment has an effect that particles are facilitated to be dispersedin the solvent, and the aggregating particles are re-dispersed. Inaddition, amorphous carbon may also be produced on the surface ofsilicon particle by irradiating ultrasonic wave energy on the slurry inwhich silicon particles are dispersed in a solution of coal-tar pitch,petroleum pitch, and a polycyclic aromatic compound, and carbon-coveredsilicon particles may be formed.

When the concentration of a solvent in the slurry obtained in the thirdprocess is too high, a slurry having an appropriate viscosity may beobtained by adjusting the amount of solvent by a method such asdesolvation by pressurization and centrifuge during the fourth process.

Additionally, a firing (calcination) process of obtaining a negativeelectrode active material by drying and calcining the slurry may befurther included, and from this, it is possible to obtain a siliconparticle having a surface layer of the amorphous carbon according to thefirst exemplary embodiment, or a negative electrode active material ofthe silicon particle-carbon composite according to the second exemplaryembodiment (a calcination process). The calcination process may beperformed under reduced pressure or under an inert atmosphere. The inertatmosphere may be a nitrogen gas atmosphere or an argon gas atmosphere.When the calcination process is performed under reduced pressure, theorganic compound present in the slurry may be more easily decomposed,and thus, may be volatilized. When the calcination is performed under aninert atmosphere, oxidation may be prevented, thereby contributing tothe enhancement of electronic conductivity of the negative electrode.The calcination may be performed at a temperature at which all theorganic materials in the slurry may be volatilized and removed.

Referring to FIG. 5, the fourth exemplary embodiment of the presentinvention may be a conductive composition for a secondary battery, inwhich carbons 121 to 123 are dispersed in a non-protic solvent 124having no proton donor ability. The conductive composition for asecondary battery according to the present exemplary embodiment may beused as a conductive auxiliary agent which may complement electronicconductivity of the negative electrode. The conductive auxiliary agentis present on the surface of particle of the negative electrode activematerial, and thus connects the negative electrode active material tothe negative electrode active material, and accordingly, electronicconductivity among the negative active materials is increased by theconductive auxiliary agent, and as a result, the conductive auxiliaryagent may contribute to enhancing performance of the negative electrode.Here, the description is made with reference to the negative electrode,but is not limited to the negative electrode, and may also be applied tothe positive electrode in some cases.

The non-protic solvent may include one or more selected from the groupconsisting of a hydrocarbon, an aromatic compound, a ketone-basedsolvent, an ether-based solvent, an ester-based solvent, an amide-basedsolvent, and a nitrile-based solvent. The non-protic solvent means asolvent having no proton donor ability, and when a non-protic solvent isused, a material to be ground or dispersed may be more efficientlysuppressed from being oxidized.

One or more additives selected from the group consisting of coal-tarpitch and a polycyclic aromatic compound may be added to the non-proticsolvent. The additive is not particularly limited as long as theadditive is a material which may be easily carbonized under an inertatmosphere. The additive may be carbonized, and thus may also contributeto enhancing electronic conductivity of the negative electrode.

Carbon may be added in order to enhance electronic conductivity of thenegative electrode. However, the additive need not be limited to carbon,and is not particularly limited as long as the additive is a materialwhich is excellent in electronic conductivity. The carbon may includeone or more selected from the group consisting of fibrous carbon 121, acarbon particle 122 with a graphene structure, and carbon black 123. Thecarbon may be present in various forms, and thus is not limited to thelisted materials, and may also include other forms of carbon.

The fibrous carbon 121 has an average diameter of 10 to 200 nm, theparticle 122 with a graphene structure has an average particle diameterof 10 to 200 nm, and the carbon black 123 has an average particlediameter of 10 to 80 nm, has a structure in which a plurality of primaryparticles is connected in a bead-like shape, and may have a crystalparticle size of 2 to 5 nm.

The content of carbon may be 0.05 to 1 g/L. When the content of carbonis less than 0.05 g/L, contribution to electronic conductivity is so lowthat an effect of enhancing the electronic conductivity of the negativeelectrode is minimal, and when the content thereof is more than 1.0 g/L,the content of carbon is so high that the fraction of the negativeelectrode active material in the entire negative electrode is low, andaccordingly, the function as a negative electrode active material maydeteriorate.

Instead of the non-protic solvent, a protic solvent having proton donorability may be used. In this case, the protic solvent may include one ormore selected from the group consisting of water and alcohol, and one ormore additives selected from the group consisting of polyvinylpyrrolidone, a carboxymethylcellulose sodium salt, and sodium cholatemay be added to the protic solvent.

The fifth exemplary embodiment of the present invention may be a methodfor manufacturing a conductive composition for a secondary battery, themethod including: a process of mixing an additive with a non-proticsolvent to prepare a solution; and a dispersing process of adding carbonto the solution to disperse the material.

The dispersing process may be performed by a beads mill.

The non-protic solvent may include one or more selected from the groupconsisting of a hydrocarbon, an aromatic compound, a ketone-basedsolvent, an ether-based solvent, an ester-based solvent, an amide-basedsolvent, and a nitrile-based solvent, and one or more additives selectedfrom the group consisting of coal-tar pitch and a polycyclic aromaticcompound may be added to the non-protic solvent.

The carbon may include one or more selected from the group consisting offibrous carbon, a carbon particle with a graphene structure, and carbonblack, the fibrous carbon has an average particle of 10 to 200 nm, theparticle with a graphene structure has an average particle diameter of10 to 200 nm, and the carbon black has an average particle diameter of10 to 80 nm, has a structure in which a plurality of primary particlesis connected in a bead-like shape, and may have a crystal particle sizeof 2 to 5 nm.

The content of carbon may be 0.05 to 1.0 g/L.

A protic solvent may be used instead of the non-protic solvent, and inthis case, the protic solvent may include one or more selected from thegroup consisting of water and alcohol, and one or more additivesselected from the group consisting of polyvinyl pyrrolidone, acarboxymethylcellulose sodium salt, and sodium cholate may be added tothe protic solvent.

In the present exemplary embodiment, the description of the solvent, theadditive, the carbon, the dispersing process and the like is the same asthat of the dry grinding process and the wet grinding and dispersingprocess in the third exemplary embodiment. However, the presentexemplary embodiment is different from the third exemplary embodiment inthat the present exemplary embodiment is not subjected to calcinationprocess.

The sixth exemplary embodiment of the present invention may be anegative electrode material including a negative electrode activematerial, a conductive agent, a binder, and a solvent. The negativeelectrode material may be a slurry in which the negative electrodeactive material, the conductive agent, the binder, and the solvent aredispersed by a ball mill. The negative electrode active material mayinclude one or more of the negative electrode active material in thefirst exemplary embodiment, and the negative electrode active materialin the second exemplary embodiment. The conductive agent may include theconductive composition in the fourth exemplary embodiment. As thebinder, it is possible to use polyvinylidene fluoride (PVDF) dissolvedin N-methyl-2-pyrrolidone (NMP), styrene-butadiene rubber (SBR)dissolved in water, or carboxymethyl cellulose (CMC).

A seventh exemplary embodiment of the present invention may be a methodfor manufacturing a negative electrode material for a second battery,the method including: preparing a negative electrode active material;preparing a conductive agent; and manufacturing a slurry by mixing thenegative electrode active material, the conductive agent and a binderwith a solvent.

First, a negative electrode active material in a slurry state may beprepared according to the third exemplary embodiment. The negativeelectrode active material may be the negative electrode active materialin the first exemplary embodiment, or may include the negative electrodeactive material in the second exemplary embodiment. Here, thedescription of the solvent, the additive, the carbon, the dispersingprocess and the like is the same as that of the dry grinding process andthe wet grinding and dispersing process in the third exemplaryembodiment. However, the present exemplary embodiment is different fromthe third exemplary embodiment in that the present exemplary embodimentis not subjected to calcination process.

Next, a conductive agent (a conductive composition) may be preparedaccording to the fifth exemplary embodiment. Next, a negative electrodematerial in a slurry state may be prepared by mixing a negativeelectrode active material in a slurry state with a conductive agent anda binder, and then uniformly dispersing the conductive agent and thebinder through a method such as milling. As the binder, it is possibleto use polyvinylidene fluoride (PVDF) dissolved inN-methyl-2-pyrrolidone (NMP), styrene-butadiene rubber (SBR) dissolvedin water, or carboxymethyl cellulose (CMC). In the present exemplaryembodiment, the description of the negative electrode active material,the conductive agent, the solvent, the binder and the like is the sameas that of the previous exemplary embodiment.

Referring to FIG. 6, an eighth exemplary embodiment of the presentinvention may be a negative electrode structure 305 including aconductive metal (a current collector) 300 and a negative electrodematerial layer 304 formed on the conductive metal 300. The negativeelectrode material layer 304 may include the negative electrode materialin the sixth exemplary embodiment. The negative electrode structure 305may be formed by applying a negative electrode material on the currentcollector 300, and then drying the negative electrode material to removethe solvent. The negative electrode material layer 304 may be in a statewhere the negative electrode active material 304 is bonded to theconductive agent 303 by the binder 302. The current collector 300 is notparticularly limited as long as the current collector 300 is a metalwhich has electronic conductivity, and is chemically stable to theelectrolytic solution and the like, and copper, aluminum and the likemay be specifically used.

Referring to FIG. 7, the ninth exemplary embodiment of the presentinvention may be a method for manufacturing a negative electrodestructure, the method including: preparing a negative electrode materialaccording to the seventh exemplary embodiment (first to fourthprocesses);

applying the prepared negative electrode material on the conductivemetal; and subjecting the applied negative electrode material to heattreatment (fifth process).

First, a negative electrode material (a slurry for an electrode coating)may be prepared according to the seventh exemplary embodiment (first tofourth processes). That is, a negative electrode material (a slurry foran electrode coating) in a slurry state may be prepared (a fourthprocess) by preparing a negative electrode active material in a slurrystate according to the third exemplary embodiment (first to thirdprocesses, here, the description of the solvent, the additive, thecarbon, the dispersing process and the like is the same as that of thedry grinding process and the wet grinding and dispersing process in thethird exemplary embodiment. However, the present exemplary embodiment isdifferent from the third exemplary embodiment in that the presentexemplary embodiment is not subjected to calcination process), preparingthe conductive agent (a conductive composition) according to the fifthexemplary embodiment, mixing a conductive agent and a binder with thenegative electrode active material in the slurry state, and uniformlydispersing the conductive agent and the binder by a method such as abeads mill. In the fourth process, a slurry for forming an electrodelayer may be manufactured by at least adding a binder or adding aconductive auxiliary agent or a solvent of a binder to a slurry of thesilicon particles obtained in the third process.

The binder to be added in the fourth process is preferably a polymerselected from polyamic acid, polyimide, polyamideimide, and an epoxyresin when the solvent of the slurry is a non-protic solvent. When thesolvent of the slurry is a protic solvent, it is preferred to use one ormore polymers selected from a carboxymethylcellulose sodium salt,polyvinyl alcohol, and chitosan, and it is more preferred to cause thecross-linking reaction to induce crosslinking between polymers. Further,in the fourth process, a conductive auxiliary agent and a solvent foradjusting the viscosity may be added to a slurry for forming anelectrode, and as the conductive auxiliary agent, it is possible to useone or more selected from the group consisting of graphite, graphene,carbon nanotube, carbon nanofiber, and amorphous carbon. The descriptionof the negative electrode active material, the conductive agent, thesolvent, the binder and the like is the same as that of the previousexemplary embodiment.

Next, a negative electrode structure may be manufactured by applying anegative electrode material (a slurry for an electrode coating) on acurrent collector at a predetermined thickness using a doctor blademethod, drying the current collector to which the negative electrodematerial is applied, and performing heat treatment thereon. The dryingtemperature is preferably in a range of 80 to 120° in the atmosphere. Anorganic material such as a binder may be volatilized and removed whilebeing subjected to heat treatment process. The heat treatmenttemperature is determined by the boiling point of the binder and thesolvent. The heat treatment at 120° C. or more needs to be performedunder vacuum (reduced pressure), or under inert gas atmosphere. Whenpolyamic acid or polyamideimide of a precursor of polyimide is used asthe binder, and N-methyl-2-pyrrolidone is used as the solvent, atemperature range of 180 to 350° C. is preferred, and the atmosphere ispreferably an atmosphere under reduced pressure or an inert atmosphere.As a gas for creating the inert atmosphere, argon, nitrogen, or heliumis preferred, but argon is more preferred.

Referring to FIG. 8, a tenth exemplary embodiment of the presentinvention may be a power storage device including the negative electrodestructure 402 of the eighth exemplary embodiment, a separator 403, apositive electrode structure 405, and current collectors 401 and 404.Representative examples of the power storage device include a lithiumsecondary battery, and here, there are types such as a coin-type, alaminate-type and a cylinder-type. FIGS. 9 to 11 illustrate across-sectional views of coin-type, laminate-type, and cylinder-typelithium secondary batteries, respectively. As the separator, thepositive electrode structure, and the current collector, those generallyused may be adopted.

The eleventh exemplary embodiment of the present invention may be amethod for manufacturing a power storage device (a secondary battery),the method including: preparing a negative electrode structure accordingto the ninth exemplary embodiment; and stacking the prepared negativeelectrode structure, a separator, and the positive electrode structure.The present exemplary embodiment describes a stack-type structure, butthe present invention is not limited thereto, and may be applied to thewinding-type, the coin-type and the like. The description of theseparator, the positive electrode structure, the current collector andthe like is the same as that of the previous exemplary embodiments.

Hereinafter, the present invention will be described in more detail withreference to Examples, Reference Examples, and Comparative Examples.

(Manufacture of Silicon-Carbon Composite)

EXAMPLE M1

A silicon-carbon composite was manufactured according to the flowchartof FIG. 3 and the grinding system of FIG. 4.

As a starting material, a mixed powder, in which 95 parts by weight of asilicon power and 5 parts by weight of a graphite powder were mixed, wasused. A silicon powder having a purity of 99% and an average particlediameter of 500 μm was used as the silicon powder, and a graphite powderhaving an average particle diameter of 20 μm was used as the graphitepowder.

A quantitative feeder 201 was used to introduce the starting materialinto a dry beads mill 202, and the starting material was ground into anaverage diameter of 3 μm. A dry grinding was performed under nitrogengas atmosphere by using zirconia beads having a particle diameter of 5mm.

A solid-liquid mixture tank 204 was used to mix the powder obtained fromthe dry grinding with a cyclohexane solution in which 0.02 wt % ofcoal-tar pitch was dissolved, and a mixed solution was obtained byadding 0.05 wt % of an artificial graphite having an average particlediameter of 5 μm, 0.0025 wt % of a multilayer carbon nanotube having adiameter of 150 μm, and 0.2 wt % of Li₂O—Al₂O₃—SiO₂—P₂O₅—TiO₂ which is alithium ion conductor (a lithium ion inorganic solid electrolyte) havingan average particle diameter of 5 μm.

A pump 205 was used to supply the mixed solution to a first wet beadsmill 209 filled with zirconia beads having a particle diameter of 0.5 mmvia a first circulation tank 207, and a slurry was obtained by grindingan average slurry of the starting material for 1 hour while circulatingthe average slurry.

The slurry was stored in a second circulation tank 210, supplied to asecond wet beads mill 212 filled with zirconia beads having a particlediameter of 0.03 mm, and ground. The slurry was ground for 2 hours whilebeing circulated until the average particle diameter of the startingmaterial became 400 nm, and the obtained slurry was stored in a groundproduct dispersion slurry tank 213. Subsequently, after cyclohexane wasadded to the obtained slurry to adjust the concentration of silicon to10 wt %, cyclohexane was added to the slurry obtained by allowing theresulting mixture to pass through a third circulation tank (notillustrated), supplying the mixture to a wet beads mill filled withzirconia beads having a particle diameter of 0.1 mm, and grinding theslurry for 2.5 hours while circulating the slurry, so as to adjust theconcentration of silicon to 5 wt %, the slurry was ground for 2 hourswhile being circulated by allowing the slurry to pass through thecirculation tank and supplying the slurry to a wet beads mill filledwith zirconia beads having a particle diameter of 0.05 mm, and then aslurry, in which silicon particles, carbon particles, and ion conductorparticles were dispersed, was obtained.

The ground dispersed slurry was dried in a vacuum dryer to removecyclohexane and obtain a mixture of silicon-lithium ion inorganic solidelectrolyte-graphite-coal-tar pitch.

The mixture was subjected to heat treatment under the condition of 800°C. under an argon gas atmosphere in a firing furnace, therebymanufacturing a silicon-lithium ion inorganic solid electrolyte-carboncomposite.

EXAMPLE M2

N-methyl-2-pyrrolidone as a dispersion medium was introduced into afirst circulation tank 207 and dispersed therein such that a metalsilicon powder having a purity of 99% and a particle diameter of 300 μmor less as the starting raw material has a concentration of 20 wt %, andzirconia beads having a size of 0.5 mm as a first wet beads mill 209were used to grind the resulting mixture for 1.5 hours while circulatingthe resulting mixture under a nitrogen gas atmosphere until the averageparticle diameter of the silicon particles became 400 nm. Subsequently,the obtained slurry was allowed to pass through a second circulationtank 210 and was supplied to a second wet beads mill 212 filled withzirconia beads having a particle diameter of 0.3 mm to grind the slurryfor 2.5 hours while circulating the slurry by a peristaltic pump 211.Subsequently, N-methyl-2-pyrrolidone was added to the obtained slurry,the resulting mixture was allowed to pass through a third circulationtank filled with zirconia beads having a particle diameter of 0.1 mm andwas supplied to a third wet beads mill filled with zirconia beads havinga particle diameter of 0.1 mm, and the slurry was ground for 2.5 hourswhile being circulated by adding coal-tar pitch thereto so as to have aconcentration of 0.1 wt % with respect to silicon, thereby obtaining aslurry of silicon particles having an average particle diameter of 180nm. The obtained slurry was dried to obtain an average particle diameterof 50 to 200 nm observed by scanning electron microscopy. As thescanning electron microscope, JSM-7400F manufactured by Jeol Ltd., wasused.

EXAMPLE M3

N-methyl-2-pyrrolidone was added to the slurry obtained in Example M2 toadjust the concentration of silicon to 5 wt %, and then the slurry wasallowed to pass through a circulation tank and supplied to a wet beadsmill filled with zirconia beads having a particle diameter of 0.05 mm,so as to grind the slurry for up to 3 hours while circulating theslurry, but the slurry was not ground until an average particle diameterof 110 nm or less 1 hour after the grinding began to be performed. Then,coal-tar pitch was added in an amount of 0.04 wt % based on the weightof silicon and the resulting mixture was further ground for 1 hour, andas a result, a slurry having an average particle diameter of which thesilicon particles was reduced to 65 nm could be obtained. The obtainedslurry was dried, and the particle diameters observed by scanningelectron microscopy were 30 to 100 nm, and particles having a minimum of5 nm were also observed by—transmission electron microscopy.

EXAMPLE M4

A silicon-carbon composite was obtained under the same conditions as inExample M2, except that ethyl alcohol in which 0.1 wt % of polyvinylpyrrolidone was dissolved was used as the solvent instead of anN-methyl-2-pyrrolidone solution in which 0.1 wt % of coal-tar pitch wasdissolved.

REFERENCE EXAMPLE M1

In Example M2, a silicon slurry was obtained by grinding the slurry to amedian diameter of 400 nm for 1.5 hours while circulating the slurryunder nitrogen atmosphere using zirconia beads having a size of 0.5 mmin the first wet beads mill 209.

REFERENCE EXAMPLE M2

In Example M1, coal-tar pitch was not added to cyclohexane. Asilicon-carbon composite was obtained in the same manner as in ExampleM1, except for the above matter.

COMPARATIVE EXAMPLE M1

A wet grinding process, in which a metal silicon having an averageparticle diameter of 20 μm was used as a starting material and ethanolwas used as a medium, was performed instead of performing the drygrinding process in Example M1. Subsequently, a dry silicon powder wasmanufactured by performing a dispersing process, performing thedesolvation, and then grinding the slurry.

Subsequently, a silicon-carbon composite was manufactured by dispersingthe dry silicon powder in a cyclohexane solution in which 2 wt % ofcoal-tar pitch was dissolved, drying the dispersion under reducedpressure, and then performing heat treatment at 800° C. under an argongas atmosphere.

(Evaluation of Silicon-Carbon Composite)

For the manufactured silicon-carbon composite, the size, distributionstate, and oxygen content of the silicon (Si) particles were analyzedand evaluated by using scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), energy dispersive spectrometer (EDS) relatedto the transmission electron microscopy, and electron energy lossspectrometer (EELS), and the results are shown below.

The aggregation size of the silicon particles was Comparative ExampleM1>Reference Example M1>Reference Example M2>Example M2>Example M1 inthis order.

The larger amount of SiO_(x) was Comparative Example M1>ReferenceExample M1>Example M2>Reference Example M1>Example M1 in this order.

The particle size of the silicon particles was Comparative ExampleM1>Reference Example M2>Reference Example M1>Example M2>Example M1 inthis order.

According to the results, it can be confirmed that according to themanufacturing method of the present invention, silicon may be furtherpulverized, silicon particles are uniformly dispersed in the internalstructure of the obtained silicon-carbon composite, and the oxygencontent is also small.

(Manufacture of Negative Electrode Structure)

EXAMPLE E1 and the Like

An artificial graphite having an average particle diameter of 5 μmaccording to Example M1, Example M2, Reference Example M1, ReferenceExample M2, and Comparative Example M1, a multilayer carbon nanotubehaving a diameter of 150 nm, and polyamic acid were added and mixed soas to have 26 parts by weight of silicon, 49 parts by weight of theartificial graphite, 12 parts by weight of carbon nanotube, and 12 partsby weight of the polyamic acid solid content, the mixture was kneaded,and then a slurry for forming an electrode layer was manufactured.

The slurry was applied on a copper foil having a thickness of 17 μmusing an applicator, and dried at 110° C. for 0.5 hour, the thicknessand the density was adjusted by a roll press, and a negative electrodeactive material layer having a thickness of 20 to 40 μm and a density of0.9 to 1.2 g/cm³ was formed on a copper foil current collector tomanufacture a negative electrode structure.

The negative electrode structures manufactured of the silicon-carboncomposites of Example M1, Example M2, Reference Example M1, ReferenceM2, and Comparative Example M1 were referred to as the negativeelectrode structures of Example EM1, Example EM2, Reference Example EM1,Reference Example EM2, and Comparative Example EM1, respectively.

(Evaluation of Amount of Electrochemical Lithium Inserted into NegativeElectrode Structure)

The negative electrode structure was used as a single electrode toevaluate the amount of electrochemical lithium inserted as follows.

A coin-type cell, in which each of the negative electrode structures ofExample EM1, Example EM2, Reference Example EM1, Reference Example EM2,and Comparative Example EM1 was pressed to a predetermined size and usedas a working electrode, and the metal lithium was used as a counterelectrode, was manufactured, and the amount of the insertedelectrochemical lithium of each negative electrode structure wasevaluated. The lithium electrode was manufactured by compressing a metallithium foil having a thickness of 140 μm on a nickel foil expandedmetal, and pressing the compressed product to a predetermined size.

A coin-type evaluation cell was manufactured as follows (see FIG. 9).That is, the negative electrode structure was inserted into a positiveelectrode can 507 under a dry atmosphere with a dew point of −50° C. orless, polyethylene films having a porosity of 40% with a microporousstructure at a thickness of 17 μm as a separator were disposed tooverlap each other, and a polypropylene gasket 508 was formed.Subsequently, an electrolytic solution was added dropwise to theseparator to form an ion conductor (a separator) 503. Subsequently,lithium negative electrodes 502 and 501 were overlapped with each other,and the lid was covered with a negative electrode cap 506 and caulked bya caulking device, thereby manufacturing an evaluation cell. As theelectrolytic solution, a solution, which was obtained by dissolving 1 M(mol/l) of lithium phosphate hexafluoride (LiPF₆) in a solvent in whichethylene carbonate from which moisture was sufficiently removed anddiethyl carbonate were mixed at a volume ratio of 3:7, was used.

The amount of electrochemical lithium inserted was evaluated as follows.The evaluation was performed by using the lithium electrode as anegative electrode and each working electrode as a positive electrode,discharging the battery until the cell voltage became 0.01 V, andcharging the battery until the cell voltage became 1.50 V. That is, theamount of electricity discharged was defined as the amount ofelectricity used to insert lithium, and the amount of electricitycharged was defined as the amount of electricity used to releaselithium.

For the charging and discharging, discharging-charging was performedonce at an electric current of 0.05 C and 50 times at an electriccurrent of 0.5 C, evaluations were performed on the amount of Liinserted (amount of electricity) once, the amount of Li released (amountof electricity) once, the ratio (%) of the amount of Li released to theamount of Li inserted once, the amount of Li released (amount ofelectricity) at the tenth time to the amount once, the amount of Lireleased (amount of electricity) at the fiftieth time to the tenth time,and Li insertion and desorption (or release) of a negative electrodecomposed of various active materials, and the results are shown below.

The largest amount of Li inserted once was Comparative ExampleEM1>Reference Example EM1>Reference Example EM2>Example EM2>Example EM1in this order.

The largest values for the amount of Li released once and the ratio (%)of the amount of Li released to the amount of Li inserted once wereExample EM1>Reference Example EM1>Example EM2>Reference ExampleEM2>Comparative Example EM1 in this order.

The largest values for the amount of Li released at the tenth time tothe first time and the amount of Li released at the fiftieth time to thetenth time were Example EM1>Example EM2>Reference EM1>Reference ExampleEM2>Comparative Example EM1 in this order.

Referring to the results, it can be confirmed that in overallconsideration of charging and discharging capacities and charging anddischarging repetition characteristics, the negative electrode structureaccording to the present invention has good performance.

Table 1 shows the comparison of current characteristics of the amount ofLi desorbed (capacity) in Example EM2 and Reference Example EM1.However, for the amount of Li desorbed at each current value, thecapacity at a current value of 0.05 C was standardized at 1.0.

TABLE 1 Capacity (mAh/g) in Reference Example Capacity (mAh/g) inCurrent value EM1/Capacity Example EM2/Capacity (C-rate) (mAh/g) at 0.05C (mAh/g) at 0.05 C 0.1 C 0.81 0.95 0.2 C 0.69 0.87 0.5 C 0.37 0.68 1.3C — 0.51

Referring to Table 1, it can be seen that an electrode (Example EM2),which was manufactured by using a silicon particle having an averageparticle diameter of up to 180 nm obtained by adding coal-tar pitch andgrinding the silicon particles, had good current rate characteristics ofthe amount of lithium desorbed, and had a larger amount of lithiumdesorbed at a higher current.

(Manufacture of Power Storage Device)

As a power storage device, a coin-type lithium secondary battery wasmanufactured, and the performance was evaluated. FIGS. 8 to 10illustrate a coin-type cell, a laminate or pouch cell, and acylinder-type cell as an example of the lithium secondary battery. Eachof the negative electrode structures of Example EM2, Reference ExampleEM1, Reference Example EM2, and Comparative Example EM1 was pressed to apredetermined size and used in the negative electrode, therebymanufacturing each coin-type cell. All the coin-type cells wereassembled under a dry atmosphere with a dew point of −50° C. or less.

Referring to FIG. 9, the process of manufacturing the coin cell will bedescribed. First, a positive electrode previously manufactured wasinserted in a positive electrode can 507, polyethylene films having aporosity of 40% with a microporous structure at a thickness of 17 μm asa separator were disposed to overlap each other, a polypropylene gasket508 was set, an electrolytic solution was added dropwise to theseparator to form an ion conductor (separator) 503, and then negativeelectrodes 502 and 501 were overlapped with each other, and the lid wascovered with a negative electrode cap 506 and was caulked in advance bya caulking device, thereby manufacturing a coin cell.

Each of the coin cells, which were manufactured by using, as a negativeelectrode, each negative electrode structure of Example EM2, ReferenceExample EM1, Reference Example EM2, and Comparative Example EM1 formedby using each silicon-carbon composite of Example M1, Example M2,Reference Example M1, Reference Example M2, and Comparative Example M1as a material for a negative electrode active material, was used as eachpower storage device for Example DM2, Reference Example DM1, ReferenceExample DM2, and Comparative Example DM1.

As the electrolytic solution, a solution, which was obtained bydissolving 1 M (mol/l) of lithium phosphate hexafluoride (LiPF₆) in asolvent in which ethylene carbonate from which moisture was sufficientlyremoved and diethyl carbonate were mixed at a volume ratio of 3:7, wasused.

(Manufacture of Positive Electrode Structure)

A positive electrode structure used as a positive electrode of the powerstorage device of the present invention was manufactured as follows.

A slurry for forming a positive electrode active material layer wasmanufactured by mixing 100 parts by weight of lithium nickel cobaltmanganese oxide (LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂) with 4 parts by weight ofacetylene black, adding 50 parts by weight of an N-methyl-2-pyrrolidonesolution containing 10 wt % of polyvinylidene fluoride and 50 parts byweight of N-methyl-2-pyrrolidone thereto, and dispersing the mixture bya wet beads mill.

Subsequently, the slurry was applied on an aluminum foil having athickness of 14 μm using a coater, dried at 110° C. for 1 hour, and thendried at 150° C. under further reduced pressure.

Subsequently, the thickness was adjusted by a roll press machine tomanufacture a positive electrode structure in which a positive electrodeactive material layer having a thickness of 82 μm and a density of 3.2g/cm³ was formed on an aluminum foil current collector. The positiveelectrode structure was pressed to a predetermined size, therebymanufacturing a LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ positive electrodestructure.

(Evaluation of Charging and Discharging Test)

After the power storage device was used to charge the battery until thecell voltage becomes 4.2 V at a constant current density of 0.2 C, thecharging and discharging, in which the battery was charged at a constantvoltage of 4.2 V and rested for 10 minutes, and then was dischargeduntil the cell voltage became 2.7 V at a constant current density of 0.2C and stopped for 10 minutes, was repeated three times, and thencharging and discharging characteristics were evaluated by repeatingcharging and discharging 100 times at a current density of 1 C, and theresults are shown below.

The largest amount of electricity discharged at the 100th time wasExample DM1>Example DM2>Reference Example DM1>Reference ExampleDM2>>Comparative Example DM1 in this order.

Referring to the results, it can be confirmed that in overallconsideration of charging and discharging capacities and charging anddischarging repetition characteristics, the negative electrode accordingto the present invention has good performance.

The terms used in the present invention are provided to describeparticular examples, but are not intended to limit the presentinvention. Unless otherwise explicitly indicated in the context, thesingular expression is deemed to include the meaning of a pluralexpression. The term “comprise” or “have” means the presence offeatures, numbers, steps, movements, constituent elements, orcombinations thereof, which are described in the specification, not theexclusion thereof. The present invention is not limited to theabove-described exemplary embodiments and the accompanying drawings, butis limited to the accompanying claims. Accordingly, various forms ofsubstitutions, modifications, and alterations may be made by the personskilled in the art without departing from the technical spirit of thepresent invention described in the claims, and these substitutions,modifications, and alterations are considered as being within the scopeof the present invention.

The invention claimed is:
 1. A negative electrode active material for asecondary battery, comprising: a silicon-carbon composite comprising asilicon particle and an amorphous surface layer including a carbonformed on a surface of the silicon particle; and a lithium ion solidelectrolyte particle in contact with the silicon particle, wherein thesilicon particle is in contact with the carbon, the silicon particle issuppressed from being oxidized by the amorphous surface layer, and thecarbon comprises one or more selected from the group consisting of acarbon particle with a graphene structure, fibrous carbon and carbonblack.
 2. The negative electrode active material for a secondary batteryof claim 1, wherein the lithium ion solid electrolyte is one or moreselected from the group consisting of a sulfur-based amorphouselectrolyte, a sulfur-containing glass, a lithium nitride, a materialwith a NASICON structure, a material with a garnet structure, and agermanium-phosphorus-sulfur compound and the lithium ion solidelectrolyte particle is complexed to the silicon-carbon compositethrough a dry process.
 3. The negative electrode active material for asecondary battery of claim 2, wherein the sulfur-based amorphouselectrolyte comprises Li₂S—P₂O₅.
 4. The negative electrode activematerial for a secondary battery of claim 2, wherein the lithium nitridecomprises Li₃N.
 5. The negative electrode active material for asecondary battery of claim 2, wherein the material with a NASICONstructure comprises Li_(1+x+y)Al_(x)Ti_(2-x)Si_(y)P_(y)O₁₂ (x=0.3,y=0.2).
 6. The negative electrode active material for a secondarybattery of claim 2, wherein the garnet structure comprises Li₇La₃Zr₂O₁₂.7. The negative electrode active material for a secondary battery ofclaim 2, wherein the germanium-phosphorus-sulfur compound comprisesLi₁₀GeP₂S₁₂.
 8. The negative electrode active material for a secondarybattery of claim 1, wherein the silicon-carbon composite has an averageparticle diameter of 5 to 20 μm.
 9. The negative electrode activematerial for a secondary battery of claim 1, wherein the siliconparticle has an average particle diameter of 5 to 200 nm.
 10. Thenegative electrode active material for a secondary battery of claim 1,wherein the amorphous surface layer comprises amorphous carbon.
 11. Thenegative electrode active material for a secondary battery of claim 1,wherein the amorphous surface layer has a thickness of 1 to 10 nm. 12.The negative electrode active material for a secondary battery of claim1, wherein the carbon particle with a graphene structure has an averageparticle diameter of 300 nm to 10 μm, the fibrous carbon has an averagediameter of 10 to 200 nm, the carbon black has a primary particle with abead-like connected structure, the primary particle has an averageparticle diameter of 10 to 80 nm, and the primary particle has a crystalsize of 2 to 5 nm.
 13. The negative electrode active material for asecondary battery of claim 1, wherein the carbon particle with agraphene structure comprises one or more selected from the groupconsisting of graphene and graphite, and the fibrous carbon comprisesone or more selected from the group consisting of carbon nanofiber andcarbon nanotube.
 14. A negative electrode active material for asecondary battery, comprising: a silicon-carbon composite comprising asilicon particle and an amorphous surface layer including a carbonformed on a surface of the silicon particle; and a lithium ion solidelectrolyte particle in contact with the silicon particle, the lithiumion solid electrolyte is a material with a garnet structure, wherein thesilicon particle is in contact with the carbon, the silicon particle issuppressed from being oxidized by the amorphous surface layer, and thecarbon comprises one or more selected from the group consisting of acarbon particle with a graphene structure, fibrous carbon and carbonblack.
 15. The negative electrode active material for a secondarybattery of claim 14, wherein the garnet structure comprisesLi₇La₃Zr₂O₁₂.